13.15 File Locks

This section describes record locks that are associated with the process.
There is also a different type of record lock that is associated with the
open file description instead of the process. See Open File Description Locks.

The remaining fcntl commands are used to support record
locking, which permits multiple cooperating programs to prevent each
other from simultaneously accessing parts of a file in error-prone
ways.

An exclusive or write lock gives a process exclusive access
for writing to the specified part of the file. While a write lock is in
place, no other process can lock that part of the file.

A shared or read lock prohibits any other process from
requesting a write lock on the specified part of the file. However,
other processes can request read locks.

The read and write functions do not actually check to see
whether there are any locks in place. If you want to implement a
locking protocol for a file shared by multiple processes, your application
must do explicit fcntl calls to request and clear locks at the
appropriate points.

Locks are associated with processes. A process can only have one kind
of lock set for each byte of a given file. When any file descriptor for
that file is closed by the process, all of the locks that process holds
on that file are released, even if the locks were made using other
descriptors that remain open. Likewise, locks are released when a
process exits, and are not inherited by child processes created using
fork (see Creating a Process).

When making a lock, use a struct flock to specify what kind of
lock and where. This data type and the associated macros for the
fcntl function are declared in the header file fcntl.h.

Data Type: struct flock

This structure is used with the fcntl function to describe a file
lock. It has these members:

short int l_type

Specifies the type of the lock; one of F_RDLCK, F_WRLCK, or
F_UNLCK.

short int l_whence

This corresponds to the whence argument to fseek or
lseek, and specifies what the offset is relative to. Its value
can be one of SEEK_SET, SEEK_CUR, or SEEK_END.

off_t l_start

This specifies the offset of the start of the region to which the lock
applies, and is given in bytes relative to the point specified by
l_whence member.

off_t l_len

This specifies the length of the region to be locked. A value of
0 is treated specially; it means the region extends to the end of
the file.

pid_t l_pid

This field is the process ID (see Process Creation Concepts) of the
process holding the lock. It is filled in by calling fcntl with
the F_GETLK command, but is ignored when making a lock. If the
conflicting lock is an open file description lock
(see Open File Description Locks), then this field will be set to
-1.

Macro: intF_GETLK

This macro is used as the command argument to fcntl, to
specify that it should get information about a lock. This command
requires a third argument of type struct flock * to be passed
to fcntl, so that the form of the call is:

fcntl (filedes, F_GETLK, lockp)

If there is a lock already in place that would block the lock described
by the lockp argument, information about that lock overwrites
*lockp. Existing locks are not reported if they are
compatible with making a new lock as specified. Thus, you should
specify a lock type of F_WRLCK if you want to find out about both
read and write locks, or F_RDLCK if you want to find out about
write locks only.

There might be more than one lock affecting the region specified by the
lockp argument, but fcntl only returns information about
one of them. The l_whence member of the lockp structure is
set to SEEK_SET and the l_start and l_len fields
set to identify the locked region.

If no lock applies, the only change to the lockp structure is to
update the l_type to a value of F_UNLCK.

The normal return value from fcntl with this command is an
unspecified value other than -1, which is reserved to indicate an
error. The following errno error conditions are defined for
this command:

EBADF

The filedes argument is invalid.

EINVAL

Either the lockp argument doesn’t specify valid lock information,
or the file associated with filedes doesn’t support locks.

Macro: intF_SETLK

This macro is used as the command argument to fcntl, to
specify that it should set or clear a lock. This command requires a
third argument of type struct flock * to be passed to
fcntl, so that the form of the call is:

fcntl (filedes, F_SETLK, lockp)

If the process already has a lock on any part of the region, the old lock
on that part is replaced with the new lock. You can remove a lock
by specifying a lock type of F_UNLCK.

If the lock cannot be set, fcntl returns immediately with a value
of -1. This function does not block waiting for other processes
to release locks. If fcntl succeeds, it return a value other
than -1.

The following errno error conditions are defined for this
function:

EAGAIN

EACCES

The lock cannot be set because it is blocked by an existing lock on the
file. Some systems use EAGAIN in this case, and other systems
use EACCES; your program should treat them alike, after
F_SETLK. (GNU/Linux and GNU/Hurd systems always use EAGAIN.)

EBADF

Either: the filedes argument is invalid; you requested a read lock
but the filedes is not open for read access; or, you requested a
write lock but the filedes is not open for write access.

EINVAL

Either the lockp argument doesn’t specify valid lock information,
or the file associated with filedes doesn’t support locks.

ENOLCK

The system has run out of file lock resources; there are already too
many file locks in place.

Well-designed file systems never report this error, because they have no
limitation on the number of locks. However, you must still take account
of the possibility of this error, as it could result from network access
to a file system on another machine.

Macro: intF_SETLKW

This macro is used as the command argument to fcntl, to
specify that it should set or clear a lock. It is just like the
F_SETLK command, but causes the process to block (or wait)
until the request can be specified.

This command requires a third argument of type struct flock *, as
for the F_SETLK command.

The fcntl return values and errors are the same as for the
F_SETLK command, but these additional errno error conditions
are defined for this command:

The specified region is being locked by another process. But that
process is waiting to lock a region which the current process has
locked, so waiting for the lock would result in deadlock. The system
does not guarantee that it will detect all such conditions, but it lets
you know if it notices one.

The following macros are defined for use as values for the l_type
member of the flock structure. The values are integer constants.

F_RDLCK

This macro is used to specify a read (or shared) lock.

F_WRLCK

This macro is used to specify a write (or exclusive) lock.

F_UNLCK

This macro is used to specify that the region is unlocked.

As an example of a situation where file locking is useful, consider a
program that can be run simultaneously by several different users, that
logs status information to a common file. One example of such a program
might be a game that uses a file to keep track of high scores. Another
example might be a program that records usage or accounting information
for billing purposes.

Having multiple copies of the program simultaneously writing to the
file could cause the contents of the file to become mixed up. But
you can prevent this kind of problem by setting a write lock on the
file before actually writing to the file.

If the program also needs to read the file and wants to make sure that
the contents of the file are in a consistent state, then it can also use
a read lock. While the read lock is set, no other process can lock
that part of the file for writing.

Remember that file locks are only an advisory protocol for
controlling access to a file. There is still potential for access to
the file by programs that don’t use the lock protocol.